the environment is close to 100 percent. Prolonged sweating without any
fluid intake causes dehydration and reduced sweating, which may lead to a
rise in body temperature and a heat stroke.
Another important factor that affects human comfort is heat transfer by
radiation between the body and the surrounding surfaces such as walls and
windows. The sun’s rays travel through space by radiation. You warm up in
front of a fire even if the air between you and the fire is quite cold. Likewise,
in a warm room you feel chilly if the ceiling or the wall surfaces are at a
considerably lower temperature. This is due to direct heat transfer between
your body and the surrounding surfaces by radiation. Radiant heaters are
commonly used for heating hard-to-heat places such as car repair shops.
The comfort of the human body depends primarily on three factors: the
(dry-bulb) temperature, relative humidity, and air motion (Fig. 14–19). The
temperature of the environment is the single most important index of com-
fort. Most people feel comfortable when the environment temperature is
between 22 and 27°C (72 and 80°F). The relative humidity also has a con-
siderable effect on comfort since it affects the amount of heat a body can
dissipate through evaporation. Relative humidity is a measure of air’s ability
to absorb more moisture. High relative humidity slows down heat rejection
by evaporation, and low relative humidity speeds it up. Most people prefer a
relative humidity of 40 to 60 percent.
Air motion also plays an important role in human comfort. It removes the
warm, moist air that builds up around the body and replaces it with fresh
air. Therefore, air motion improves heat rejection by both convection and
evaporation. Air motion should be strong enough to remove heat and mois-
ture from the vicinity of the body, but gentle enough to be unnoticed. Most
people feel comfortable at an airspeed of about 15 m/min. Very-high-speed
air motion causes discomfort instead of comfort. For example, an environ-
ment at 10°C (50°F) with 48 km/h winds feels as cold as an environment at
7°C (20°F) with 3 km/h winds as a result of the body-chilling effect of the
air motion (the wind-chill factor). Other factors that affect comfort are air
cleanliness, odor, noise, and radiation effect.
14–7 AIR-CONDITIONING PROCESSES
Maintaining a living space or an industrial facility at the desired temperature
and humidity requires some processes called air-conditioning processes.
These processes include simple heating(raising the temperature),simple cool-
ing(lowering the temperature),humidifying(adding moisture), and dehumidi-
fying(removing moisture). Sometimes two or more of these processes are
needed to bring the air to a desired temperature and humidity level.
Various air-conditioning processes are illustrated on the psychrometric
chart in Fig. 14–20. Notice that simple heating and cooling processes appear
as horizontal lines on this chart since the moisture content of the air remains
constant (vconstant) during these processes. Air is commonly heated
and humidified in winter and cooled and dehumidified in summer. Notice
how these processes appear on the psychrometric chart.
Chapter 14 | 72923 °C
f = 50%
Air motion
15 m/minFIGURE 14–19
A comfortable environment.
© Reprinted with special permission of King
Features Syndicate.Cooling
HeatingHumidifyingDehumidifyingCooling and
dehumidifyingHeating and
humidifyingFIGURE 14–20
Various air-conditioning processes.